Wet oxidation treatment is well suited to high strength liquors or wastewaters. Not all material needs to be considered "waste" to be treated by wet oxidation. This liquid phase oxidation treatment at elevated temperature and pressure with an oxygen gas source destroys a substantial portion of the pollutants in these wastewaters. The oxidation reactions which degrade the pollutants generate significant heat energy. The liquid phase in the wet oxidation system acts as a heat sink and absorbs this energy by conversion of liquid water to steam, providing one means of temperature control in the process.
These high strength liquors or wastewaters may also contain high concentrations of dissolved salts and/or suspended solids. Additional salts and/or solids may be generated by the wet oxidation reactions of the pollutants in the wastewater. Evaporation of the liquid phase, described above, also increases the concentration of dissolved salts and/or suspended solids in the wet oxidation system.
High solids concentrations in the wet oxidation system can produce a variety of negative effects. If the solubility of the dissolved salts is exceeded, the system can be plugged with scaling products, crystals or other forms of solids.
High solids concentrations also can interfere with the transfer of oxygen from the gas phase to the liquid phase and thus inhibit the wet oxidation process. Additionally, higher solids concentrations tends to increase the corrosive characteristics of the liquor or waste being processed. In all cases, the undesirable effects are magnified as the solids concentration in the system increases.
As a companion to pollutant destruction, it may be desirable to recover the dissolved/suspended solids from the wet oxidation treated liquor or wastewater. In this case a high solids content effluent is preferred. Thus, treating a liquor or wastewater with high pollutant content and high solids content may result in opposed objectives, and can be quite difficult. The usual solution to the operational difficulties encountered for this type of liquor or wastewater is to dilute the incoming feed liquor or wastewater to reduce both the pollutant and solids concentrations. This is not a good approach when one treatment goal is to produce a high solids content effluent product. The problem then becomes how to dilute the pollutant concentration without diluting the solids concentration, and at the same time avoid the formation of solid scales and/or crystals, and minimize the effects of solids concentration on oxygen transfer and materials of construction factors.
It is an objective of the instant invention to treat a high pollutant concentration liquor or wastewater by wet oxidation to remove a substantial portion of the pollutants. It is also an objective to minimize the negative effects of high solids concentrations in the wet oxidation system. It is a further objective to produce an effluent stream of high solids content, for recovery or disposal of said solids within that effluent stream.
Japanese patent publication JP 55022367 assigned to Asahi Chemical Industries KK discloses a wet oxidation catalyzed with copper salt with an ammonium salt present where a portion of the oxidized liquor is recycled to dilute the COD concentration of the feed waste liquor. The copper catalyst is recovered from the effluent for reuse in the process.
Othmer in U.S. Pat. No. 4,251,227 describes the wet oxidation treatment of wet solid wastes or low grade fuels to produce a low moisture fuel for various uses, plus several other effluent liquid streams which may or may not be recycled to the wet oxidation process.
Soukup et al. in U.S. Pat No. 4,330,038 discloses a wet oxidation process for steam flooding of oil-bearing formations where water from the formation and condensate from the wet oxidation system are recycled to the process to further treat these liquid streams.
Copa et al. in U.S. Pat. No. 5,240,619 describe several two-stage wet oxidation flow schemes where the vapor phase and liquid phase from a subcritical wet oxidation reactor are routed to an additional supercritical wet oxidation treatment step.
None of the above references have addressed the problem of producing a single high solids content effluent from the wet oxidation treatment of a high pollutant concentration liquor or wastewater. Applicant has devised such a process as described below.